In the field of sensor applications, components are often used in practice which include micromechanical components such as a membrane, for example, in addition to an integrated circuit. For cost reasons and considerations regarding miniaturization of such components, it is advantageous to structure the membrane before the actual semiconductor process. In a conventional manufacturing method known in the art, for example, a porous area is first produced in the surface of a silicon substrate. After growing an epitaxial silicon layer on the substrate surface, the porous area is transformed into a cavity at high temperatures; this cavity is spanned and sealed by the thin epitaxial layer. The cavity usually contains a vacuum. The epitaxial silicon layer is used not only for sealing the cavity, but it also functions as a base layer for circuit elements, which are subsequently produced by deposition or diffusion, for example. In general, high-temperature steps, carried out at a process pressure of up to one atmosphere, are required for producing the circuit elements.
It has been found that the membrane of a component, which, as described previously, is exposed prior to processing of the circuit elements, has a deflection toward the cavity even at room temperature and vacuum (p=0 bar).
This effect is at least in part due to the material properties of the silicon layer. Although silicon has an almost perfect elastic behavior at room temperature, it becomes softer and plastically deformable if the ambient temperature rises over 500 K to 800 K. Since some of the deposition processes that are required for manufacturing the circuit elements take place under pressure and at significantly higher temperatures of over 1000 K, while the cavity has vacuum in it, the thin silicon layer over the cavity is deformed inward toward the cavity. In this case, the edge of the cavity, i.e., the membrane, is under tensile stress, while the center of the membrane is under compressive stress. At high process temperatures the silicon layer is plastically deformed, at least at the points of high tensile stress. This plastic deformation remains preserved even at room temperature and without external pressure.
In addition, the elastic deformation of the thin silicon layer is “frozen in” by deposition of further layers under pressure. Even the first layer deposited on the deformed silicon layer is formed on a surface that is larger than that of the undeflected silicon layer. Therefore, this deposited layer functions as a wedge, which prevents the full relaxation of the silicon layer when the process pressure is removed. This effect is further reinforced by the production of further layers. Mechanical stresses acting in the membrane under vacuum (p=0 bar) are thus “frozen in.”
The circuit elements situated on or in the membrane are usually components of an analyzer circuit, such as a Wheatstone bridge circuit, for example, with which, for example, the pressure that a corresponding deflection of the membrane causes may be determined. The mechanical stresses “frozen in” in the membrane, which remain effective even at an ambient pressure of p=0 bar, often result in practical problems in signal detection and analysis. In the case of an integrated bridge circuit, the mechanical stresses “frozen in” in the membrane result in an undesirable stress offset shift.